The usefulness and novelty of the present invention is illustrated by considering typical requirements and challenges in the design of a bar code reading system.
In a beam scanning type barcode reader a small spot of light of a specific wavelength, usually from a laser, is rapidly scanned across the barcode target to be read. For purposes of definition the means of scanning light shall be called a scan element. As the spot of light traverses the alternating dark and light bands of the code it is absorbed and diffusely reflected.
The barcoded information is radiated as diffuse light reflected in all directions from the target in the form of high frequency light pulses. A portion of this diffuse light returns to the bar code reader, whereupon the bar code reader collects and processes it. Unwanted stray light of wavelengths of light different from that emitted by the laser are filtered out to improve signal to noise ratio. The collected light signal is directed onto a photoelectric converter, usually a photodiode, whereupon it is transformed into electronic signals, further amplified, and shaped into digital pulses suitable for computer processing.
A major concern in the design of a beam scanning type bar code reader is that the amount of information bearing light reaching the bar code reader is very small. Usually the laser output power is limited to only one milliwatt for safety reasons and the barcode target may be located several feet from the barcode reader. Also the paper upon which the barcode is printed often is selected for eye appeal rather than its light reflecting characteristic and thus may be far from an ideal reflector in the areas which are not printed with dark bars. In consideration of these factors it is clear that the light returned to the photoelectric converter is indeed very weak.
In order to reliably process such weak information signals it is incumbent upon the light collection means of the barcode reader to collect as much of the returned light as possible and to concentrate it upon the photoelectric converter with the highest possible signal to noise ratio.
Another critical consideration in the processing of the light signal returned from the barcode is its pulse rate or frequency. This frequency may be quite high and require very high speed response of the photodiode as well as wide bandwidth amplification.
For example it is quite common in industrial barcode scanners to scan the spot back and forth through a thirty degree angle at a rate of two hundred scans per second.
It is readily calculated at only one foot away from the scan element the average spot speed will be over one hundred feet per second. In actual practice however the spot may not have a constant speed and will reach peak speeds of several times the average speed. As this spot moves across bars and spaces as small as five thousandths of an inch it may be shown that the frequency of the light pulses correspond to modulation frequencies of several hundred kilohertz. Some industrial scanners run at a thousand scans per second or more and hence may produce return light pulses which reach several megahertz. Thus the photoelectric converter must have a very high speed detection capability.
From the foregoing discussion it may be concluded that the farther away the target is from the scan element the weaker will be the returned light signal and the higher its frequency will be. These considerations complicate the signal handling requirements imposed upon a photo detection system. In fact the requirements may be in conflict as is shown below.
It is well known that a photodiode used as a photoelectric converter must have low junction capacitance in order to meet the needs of high speed signal detection. However junction capacitance increases in proportion to the area of the junction. Yet large junctions increase light gathering capacity and are desirable where weak signal detection is necessary. Also, in order to obtain a high signal to noise ratio, photodiodes must also possess low dark current leakage and high shunt resistance. These properties all get worse the larger the junction area of the photodiode is. When both high speed and great sensitivity are needed as in the case of barcode reading or LAN reception the two requirements for speed and sensitivity are in conflict. In order to lower junction capacitance, reverse bias voltage may be applied to the photodiode. This however has the drawback of increasing noise from the photodiode which tends to diminish the signal to noise ratio. It is not always possible to sacrifice signal to noise ratio where signals are already weak. More expensive PIN photodiodes have lower junction capacitance than the PN junction photodiodes but are much more expensive per unit area. Thus a designer must choose a high priced premium photodiode to begin with and make compromises from there.
In low speed scanners with scan rates on the order of 36 scans per second it has been feasible to use two large area photodiodes to collect light. U.S. Pat. No. 4,387,297 describes such a low speed scanner which uses a pair of photo diodes to collect light directly. This approach is expensive and consumes considerable space due to mounting requirements for the photodiodes and is inadequate for higher speed scanners running at hundreds of scans per second.
In order to improve the signal to noise ratio and keep the junction size down, lenses have been used to collect and concentrate light upon a photodiode surface. A positive lens or combinations of lenses are sometimes used to focus light on a photodiode but these are expensive and suffer from other drawbacks as will be seen.
U.S. Pat. No. 4,797,551 describes a multiple lens and mirror system for light concentration. This approach improves signal to noise ratio but is characterized by slow speed since the photodiode must be large enough to roughly accommodate the entire image of the target area scanned.
Another approach to light collecting is to use a large mirror which simultaneously scans and tracks the outgoing beam. This mirror with its large surface area serves as a light collector for the reflected light which is in turn reflected off a concave mirror and focused upon a photodiode. This approach with its many parts and optical layout is not compact. It also requires that the image of the entire target as viewed by the tracking mirror be imaged upon a photodiode which still demands a considerable area diode with its attendant junction problems.
U.S. Pat. No. 4,816,660 describes the use of a large concave second surface mirror with a small first surface mirror attached to it for scanning the outgoing light beam. The concave second surface mirror moves along with the first surface scanning mirror so as to track the spot. The mirrors are attached to the shaft of a stepper motor which serves as a scan element in order to move them.
Returned light intercepted by the concave collector mirror images the light onto a photodiode. This system saves some parts but since the concave collection mirror must move, its mass places an inertial load on the scan element tending to make this system inherently slow and considerable power is required to dither the mirror. Another shortcoming of this approach is the lack of ruggedness of the mirror motor combination. In portable applications this system is highly vulnerable to breakage upon dropping and to shock.
U.S. Pat. No. 4,794,237 uses a spinning holographic disc to scan and collect light and to image it onto a suitable photoelectric converter, as well as to filter out unwanted wavelengths. But holographic systems are bulky, expense and fragile.
Presently very high density two dimensional bar codes are being introduced into commercial use. These are read by high speed raster scan techniques. To read these codes the beam is swept rapidly back and forth across them and at the same time slowly down them in zig-zag or raster fashion. High speed photodetection and signal processing is necessary to read two dimensional bar code since scan rates are about an order of magnitude higher than for straight line scanning. For these scan systems it is desirable to use very small mirrors with low inertia in order to scan at high speeds but small mirrors collect only small amounts of light.